The prosthesis should be the same maybe some differences in brand but it's not like you're buying a cheap 2 door hatchback knee versus a Porsche. I'm unfamiliar with such a discrepancy and it might be more due to demographics than anything else. Around here they're all roughly the same. Some private practice places are a bit cheaper because you don't have a big hospital charge associated with everything. Those patients do just fine. So my advice would be first to look at a private group that does surgery out of their own "hospital." If you're considering going out of state for a procedure research the physician and maybe just ask them straight out what the difference is and why can they be cheaper. Often it's because they don't have the overhead bigger hospital groups have.
FWIW... she's at a great age for a knee replacement. Young enough to get a "second go" at living life and being active. And if she needs another one in 15-20 years, she's still doing just fine. The problem is people wait way too long and then their body is so broken down every where else (hips- need those done then!) and they never really get back to being very active... at that point they are just wanting to be functional. Yes, prolotherapy might buy her 5 years and then it's a Medicare thing, so that's always nice. I think you're asking great questions and doing some great research in the best decision.